1. Field of the Invention
The subject invention relates to burners for gas turbines, and more particularly, to burners adapted to stabilize engine combustion, and still further, to burners that use a pilot combustor to provide combustion products (e.g., heat and free radicals) to stabilize the main lean premixed combustion.
2. Background of the Related Art
Gas turbines are employed in a variety of applications including electric power generation, military and commercial aviation, pipeline transmission and marine transportation. In a gas turbine engine, fuel and air are provided to a burner chamber where they are mixed and ignited by a flame, thereby initiating combustion. The major problems associated with the combustion process in gas turbine engines, in addition to thermal efficiency and proper mixing of the fuel and air, are flame stabilization, the elimination of pulsations and noise, and the control of polluting emissions, especially nitrogen oxides (NOx).
The combustion process requires heat to be added to the fuel-air mixture in order to initiate the reaction. Once the reaction has started, the heat released by the combustion can be used to initiate the reaction itself and the process becomes self-sustaining. However, some mechanism must be used to transport the heat from the combustion back upstream to the ignition point. Alternatively, where the reaction is not self-sustaining, heat and/or free radicals must be provided from a separate source, such as, a heated catalytic metal surface, or a separate pilot flame. Any combination of these methods, as well as, other methods, can also be used to provide the necessary heat to initiate combustion.
The most common self-sustaining combustion process used in gas turbine engines utilizes swirling air flows that recirculate the combustion products and transport the hot gases and free radicals produced by the previously reacted fuel and air back upstream to initiate the combustion of the freshly mixed fuel and air. A prior art swirl-stabilized burner is illustrated in FIG. 1 and is designate by reference numeral 100. Generally, with swirl-stabilized combustion, the center main recirculation zone is the dominant source of recirculated hot gases and free radicals that are transported upstream to stabilize the combustion. When the combustion process becomes very lean, and therefore, little heat is released from the combustion and the amount of heat and free radicals that are transported back upstream is insufficient to assure that combustion is initiated and sustained. The low temperatures of the lean combustion products produce low equilibrium levels of free radicals. The low temperature of the combustion products caused by the lean combustion also results in a low free radical production rate when the recirculated combustion products mix with the fresh un-reacted premixed fuel and air. Under these conditions the induction time required to initiate combustion becomes excessive and the flame blows away, or becomes unstable and fluctuates in intensity.
The basic problem with lean premixed combustion systems used to produce low NOx emissions is that the fuel-air mixture must be so lean in order to have the flame temperature sufficiently low to prevent NOx production that under many operating conditions the combustion may not produce sufficient heat to be self-sustaining. An auxiliary source of heat and free radical must be used to sustain combustion. If an auxiliary pilot at high temperatures (close to stoichiometric) is used, it will stabilize the lean main flame, but it will produce substantial NOx emissions.
Purely thermally initiated combustion starts the combustion process by pyrolyzing fuel at high temperatures to produce active free radicals. Initially this occurs with very low fuel consumption and no measurable temperature rise. Through chain branching reaction mechanisms the initially produced free radicals create an exponentially increasing pool of free radicals. Eventually the radical pool becomes sufficiently large to consume a significant amount of fuel, leading to rapid ignition (Wamatz). The time it takes for this pool of free radicals to increase sufficiently enough to cause the ignition is the “Ignition-Delay Time” or the “Induction Time”. When the initial temperature is increased the production rate of free radicals is increased at an exponential rate and the induction time for initiation of combustion is reduced. If the initial temperature is less than the auto-ignition temperature, no ignition will occur for any time period. Free radicals, as well as, hot gases are contained in the combustion products that are mixed with the fresh premixed fuel-air mixture, in order to initiate combustion. These previously generated free radicals can significantly reduce the induction time for combustion. If the entrained free radicals are in sufficient quantity, rapid combustion initiation will occur at lower temperatures, which would otherwise have long induction times without the entrained free radicals. Stable combustion requires rapid initiation of combustion of the premixed fuel and air immediately after being mixed with the hot products of the previously burnt fuel.
In view of the foregoing, a need exists for an improved burner, which reduces NOx emissions while maintaining a stable combustion process.